Corresponding heating elements have long been known. As already mentioned they comprise at least one tube through which gas is to flow and which is open at both ends for the purposes of the flow of gas therethrough, wherein arranged in the tube is a heating wire along which the gas flows and is heated by the direct contact with the heating wire.
Usually the heating wires are in the form of fine wires which are wound in a spiral configuration and whose cross-section is very much smaller than the tube cross-section and which have current passing therethrough and are thereby heated. The electrical energy converted into heat by the heating wire obviously depends on the available electrical voltage and the resistance of corresponding heating wires, in which respect to achieve desired resistance values the length of a spiral-wound wire can be correspondingly adapted or a plurality of corresponding heating wires can be connected in parallel or also in series. It will be appreciated that in that case the heat energy transferred to the gas flowing along the heating wire depends on the maximum temperature that the heating wire achieves, the flow resistance and the surface area available for heat exchange, as well as the precise flow conditions in the heating element. The maximum gas temperatures which can be regularly achieved in practice in continuous operation with such process heaters are of the order of magnitude of 700° C.
Admittedly heating elements or process heaters are also occasionally offered, which make it possible to produce higher gas temperatures of up to about 900° C., but those have only extremely short service lives. With the gas flow rates required for many processes the heating wire itself necessarily always involves a temperature which is more or less markedly above the gas temperature, in which respect even the smallest non-homogeneities in the heating wire or in the cross-section thereof or also unfavourable local flow conditions and turbulence phenomena can have the result that some portions of the heating wire heat up more greatly than the remaining part, which then rapidly results in fracture and failure of the heating wires. As the heating wire typically contains small amounts of aluminium contact with oxygen initially leads to the formation of a protective aluminium oxide layer around the wire. After consumption of the aluminium component however other alloy constituents like iron and chromium react with the oxygen, which generally signifies the end of the operating life of the heating wire. Other chemical reactions in respect of the process gas which is hot or which is to be heated with the material of the heating wire can also speed up failure or fracture of the heating wires. Small irregularities in the material or the cross-section of the heating wire due to chemical changes quickly lead to local overheating of the heating wire and fracture. As the stability of the very thin coiled heating wires is very low, in particular at high temperatures, the heating coils in a vertical tube can easily collapse into themselves, thereby giving rise to short-circuits which also reduce the operating life of such coiled wires. Such a failure due to overheating, in particular local overheating, occurs correspondingly more easily, the smaller the cross-section or diameter of the heating wires. On the other hand however a large surface area-to-volume ratio of the heating wires is deemed to be advantageous for effective transfer of the heat energy produced in the heating wire to the gas flowing past same, so that hitherto the short service life of such heating elements has been accepted if the aim is to achieve gas temperatures in the region of 900° C. or above.
Process heaters and heating elements which produce gas temperatures of 900° C. or even still higher usually only have a service life of a few hours for the above-mentioned reasons.